This project involves the development and use of a new kind of microscopic technique, photoelectron microscopy, based on the photoelectric effect. A typical experiment consists of placing a specimen in a vacuum chamber and subjecting it to ultraviolet light. If the energy of the light source is sufficiently high, the sample surface can emit electrons (photoionize) as well as fluoresce. The photoelectrons are then accelerated, passed through a series of electron lenses and focused onto the phosphor screen (the image plane). This is an entirely different kind of experiment than conventional or scanning electron microscopy. However, photoelectron microscopy can be thought of as an extension of fluorescence microscopy using electrons instead of photons. The proposed new technique photoelectron microscopy, is potentially a powerful tool for the study of biological surfaces, including the surfaces of cells in tissue culture. In the initial three-year period the feasibility of this approach was demonstrated. A prototype instrument with magnification range x10- 200 was constructed and the first photoelectron images of organic and biological surfaces were reported. During the fourth year, the depth resolution was measured and found to be on the order of 15 A for the test compound, metal-free phthalocyanine. This is a remarkably high depth resolution compared to other known microscopic techniques and will permit mapping of membrane surfaces without interference from the cytoplasmic contents. The thrust during 1974 will be to construct a photoelectron microscope of intermediate power (x200- x2,000) and to use this instrument to develop methodology such as specimen preparation and choice of support films. The long term goal of this project is to map the distributions of specific proteins in biological membranes.